1. Field of the Invention
The present invention relates to a semiconductor ceramic composition having a positive resistive temperature, which is used for a PTC thermistor, a PTC heater, a PTC switch, a temperature detector and the like, and a process for producing the same.
2. Description of the Related Art
As materials showing a PTCR characteristic (Positive Temperature Coefficient of Resistivity), compositions in which various semiconductor dopants are added to BaTiO3 have been conventionally proposed. These compositions have a Curie temperature around 120° C. Depending upon the use, it becomes necessary for these compositions to shift the Curie temperature thereof.
It has been proposed to shift the Curie temperature by adding, for example, SrTiO3 to BaTiO3. However, the Curie temperature shifts only to a negative direction and does not shift to a positive direction in this case. Currently, only PbTiO3 is known as an additive material for shifting the Curie temperature to a positive direction. However, since PbTiO3 contains an element that causes environmental pollution, a material using no PbTiO3 has been demanded in recent years.
Regarding BaTiO3 semiconductor ceramics, there is proposed a process for producing a BaTiO3 semiconductor ceramic by adding one or more of Nb, Ta and rare earth elements to a composition having a structure of Ba1-2x(BiNa)xTiO3, wherein a part of Ba in BaTiO3 in which no PbTiO3 is used is substituted with Bi—Na and x is controlled to be in a range of 0<x≦0.15, sintering the composition in nitrogen, and then subjecting the composition to a heat treatment in an oxidizing atmosphere, for the purpose of restraining the reduction in the temperature coefficient of resistance due to Pb substitution and for the purpose of reducing the voltage dependence to thereby enhance the producibility and the reliability of the semiconductor ceramic (see, Patent Document 1).
Patent Document 1: JP-A-56-169301